Multilayer cathode for organic light-emitting device

ABSTRACT

An organic light-emitting device wherein the cathode ( 4, 5 ) comprises a first layer ( 5 ) of a conducting material and a second layer ( 4 ) of a conductive material having a work function of at most 3.7 eV and wherein the second layer is substantially thinner than the first layer, having a thickness of at most 5 nm.

The field of the invention relates to organic light-emitting deviceswith efficient electron injecting electrodes.

Organic light-emitting devices (OLEDs) such as described in earlier U.S.Pat. No. 5,247,190 assigned to Cambridge Display Technology Limited, orin Van Slyke et al., U.S. Pat. No. 4,539,507, the contents of which areherein incorporated by reference and example, have great potential foruse in various display applications. Principally, an OLED consists of ananode that injects positive charge carriers, a cathode that injectsnegative charge carriers and at least one organic electroluminescentlayer sandwiched between the two electrodes. One of the key advantagesof the OLED technology is that devices can be operated at low drivevoltages, provided that suitable electro-luminescent organic layers, andelectrodes with good efficiencies for the injection of positive andnegative charge carriers, are used. Typically although not necessarilythe anode is a thin film of, for example, indium-tin-oxide (ITO), whichis a semi-transparent conductive oxide which is commercially readilyavailable already deposited on glass or plastic substrates. The organiclayer(s) is normally deposited onto the ITO-coated substrate by, forexample, evaporation, or any one of spin-coating, blade-coating,dip-coating or meniscus-coating. The final step of depositing thecathode layer onto the top organic layer is normally performed bythermal evaporation or sputtering of a suitable cathode metal. Layers ofAl, Ca or alloys of Mg:Ag or Mg:In or Al alloys are often used ascathode materials. In order to achieve good performance in OLEDs it isof great importance to optimise all individual layers, the anode, thecathode and the organic layer(s), as well as the interfaces between thelayers.

It is very often found that the electron-injection properties of thecathode are particularly important to achieve efficient deviceoperation. Due to the electronic structure of most organicelectroluminescent materials it is very often necessary to employcathode materials with a low work function to achieve efficient electroninjection and low operating voltages. Such cathodes for OLEDs aretypically alkali metals such as Li, Na, K, Rb or Cs, alkaline earthmetals such as Mg, Ca, Sr or Ba or lanthanides such as Sm, Eu, Tb or Yb.These materials tend to react very readily with oxygen and moisture andparticular care has to be taken on handling and/or during and afterdeposition onto OLEDs. Often these low work function materials aredeposited as cathode layers onto OLEDs in the form of alloys wherebyother alloy constituents stabilise the cathode layer; typical suchalloys are for example Mg:Al, Mg:In or Mg:Ag or Al:Li. When using someof these low work function elements, for instance Ca, K, Li or Sm inpure or alloy form as OLED cathode layers these elements can diffuseinto the organic layer(s) and subsequently dope the organic layer(s),cause electrical shorts or quench photo-luminescence, and thereforegenerally deteriorate device performance.

It is thus an object of the present invention to provide a structure,and method of fabrication for, an organic light-emitting device thatincorporates low work function elements as cathodes to achieve efficientinjection of negative charge carriers and low operating voltage, butminimises at least some of the problems outlined above.

According to a first aspect of the present invention there is providedan organic light-emitting device, comprising at least one layer of alight-emissive organic material arranged between an anode and a cathodefor the device, wherein the cathode comprises a first layer of aconductive material which is an opaque metallic layer of highconductivity and a second layer of a conductive material having a lowwork function arranged between the at least one layer of organicmaterial and the first layer of conductive material, wherein the secondlayer of conductive material is substantially thinner than the firstlayer of conductive material, having a thickness of at most 5 nm, andcomprises an elemental metal, an alloy or an intermetallic compoundhaving a work function of at most 3.7 eV.

Thus, the cathode is composed of a thin layer of an elemental metal,alloy or inter-metallic compound, with a work function of at most 3.7eV, but preferably less than 3.2 eV. The cathode layer has a thicknessof at most 5 nm, but preferably of between 0.5 and 2 nm thick. The thin,low work function cathode layer is preferably capped with anotherconductive layer, typically 100-500 nm thick, which provides highconductivity protection for the underlying thin, low work functionlayer, as well as environmental stability. Such a bi-layer electrodestructure according to the first aspect of the present invention forms acathode layer with efficient electron injection for an OLED with atleast one electro-luminescent organic layer between said cathode layerand an anode layer, the anode layer for injecting positive chargecarriers.

Such a structure prevents excessive doping and minimises the risk ofshorting of the device structure and quenching of theelectro-luminescence of the at least one layer of organic material. Thefirst aspect of the invention also provides a method of fabricating anorganic light-emitting device, comprising the steps of forming a cathodefor the device over a substrate, which step comprises forming a firstlayer of a conductive material of high conductivity over a substrate andforming a second layer of a conductive material having a low workfunction over the first layer of conductive material, wherein the firstlayer of conductive material is an opaque metallic layer and the secondlayer of conductive material is substantially thinner than the firstlayer of conductive material having a thickness of at most 5 nm, andcomprises an elemental metal, an alloy or an intermetallic compoundhaving a work function of at most 3.7 eV, forming at least one layer ofa light-emissive organic material over the cathode and forming an anodefor the device over the at least one layer of organic material.

The first aspect of the invention further provides a method offabricating an organic light-emitting device, comprising the steps offorming an anode for the device over a substrate, forming at least onelayer of a light-emissive material over the anode and forming a cathodefor the device over the at least one layer of organic material, whichstep comprises forming a second layer of a conductive material having alow work function over the at least one layer of organic material andforming a first layer of a conductive material of high conductivity overthe second layer of conductive material, wherein the first layer ofconductive material is an opaque metallic layer and the second layer ofconductive material is substantially thinner than the first layer ofconductive material having a thickness of at most 5 nm, and comprises anelemental metal, an alloy or an intermetallic compound having a workfunction of at most 3.7 eV.

Thus there is also provided a method of fabrication for an OLED with anefficient electron-injecting electrode in which at least oneelectro-luminescent organic layer, preferably either polymeric ormolecular, is deposited preferably onto a supportive substratepre-coated with an anode. The organic layer is coated in one embodimentby vacuum evaporation, with a thin layer of conductive, low workfunction material. This layer is at most 5 nm but preferably between 0.5and 2 nm thick, and still more preferably about 0.5 nm. This thin layeris typically, but not essentially, an alkali metal, alkaline earth metalor a lanthanide or an alloy or inter-metallic compound incorporating oneor more of said alkali metal, alkaline earth metal or lanthanideelements. The thin low work function layer is then preferably coveredwith a thick conductive layer of typically 100 to 500 nm thickness whichprovides high conductivity, protection for the underlying thin low workfunction layer and environmental stability, and which is preferablyapplied by vacuum evaporation or sputter deposition.

According to a second aspect of the present invention there is providedan organic light-emitting device, comprising at least one layer of alight-emissive organic material arranged between an anode and a cathodefor the device, wherein the cathode comprises a first layer of aconductive material which is a DC magnetron sputtered metallic layer ofhigh conductivity and a second layer of a conductive material having alow work function arranged between the at least one layer of organicmaterial and the first layer of conductive material, wherein the secondlayer of conductive material is substantially thinner than the firstlayer of conductive material.

Such a structure provides advantages similar to those discussed abovewith reference to the first aspect of the present invention.

The invention will now be described with reference to a particularexample as shown in the accompanying drawing, in which:

FIG. 1 illustrates a structure of an OLED in accordance with the presentinvention.

According to the illustrated embodiment of the invention, an OLED isformed by first forming a semi-transparent anode deposited onto atransparent supportive substrate. The substrate is, for example, a thinsheet of glass or plastic such as polyester, polycarbonate, polyimide,poly-ether-imide or the like. Referring to FIG. 1, a glass substrate 1is covered with a layer of a semi-transparent conductiveindium-tin-oxide (ITO) layer 2, typically about 150 nm thick with asheet resistance of typically≦30 Ohms/square. Although thesemi-transparent anode shown in FIG. 1 is a thin layer of conductiveoxide such as indium-tin-oxide, it may alternatively be a dopedtin-oxide or zinc-oxide.

The organic layer(s) deposited on top of the anode/substrate is/arepreferably, but not necessarily, one or more layers of anelectro-luminescent conjugated polymer such as described in earlier U.S.Pat. No. 5,247,190 assigned to Cambridge Display Tehnology Limited. Suchorganic layer is formed to a thickness typically of the order of 100 nmthick. Alternatively the organic layer(s) could be low molecular weightcompounds such as described in U.S. Pat. No. 4,539,507, or a combinationof layer(s) of conjugated polymer(s) with layer(s) of low molecularweight compound(s). In FIG. 1, the ITO layer is covered with a ca. 100nm thick layer 3 of the electroluminescent polymer poly(p-phenylenevinylene), PPV, as for example described in U.S. Pat. No. 5,247,190.

The cathode may be a thin layer of an alkali metal, alkaline earth metalor a lanthanide or an alloy or inter-metallic compound incorporating oneor more of said alkali metal, alkaline earth metal or lanthanideelements. The cathode layer is at most 5 nm but preferably between 0.5and 2 nm thick and examples for materials which may be used are Li, K,Sm, Ca or an Al:Li alloy. In the example of FIG. 1, the PPV layer 3 ispreferably covered with a 0.5 nm thick layer 4 of Li deposited by vacuumsublimation of the Li from a commercial Al:Li alloy.

The thin layer of the cathode may be sputter deposited, preferably by DCmagnetron sputtering or RF sputtering. The thin layer of the cathode mayalso be evaporated, preferably by resistive evaporation or electron beamthermal evaporation.

The thin layer of the cathode, being a conductive material comprisingeither an elemental metal, an alloy or an intermetallic compound havinga work function of at most 3.7 eV, and preferably at most 3.2 eV.

The thin layer is then preferably covered with a conductive layer of,for example, aluminium or an aluminium alloy which is typically between100 and 500 nm thick, and preferably about 100 nm. In FIG. 1, the thinlayer 4 is, without breaking vacuum, preferably covered with a 150 nmthick layer 5 of Aluminium deposited by vacuum evaporation.

The thick conductive layer may be sputtered, preferably by DC magnetronsputtering or RF sputtering. The thick layer may also be evaporated byresistive evaporation or electron-beam thermal evaporation.

Preferably, the ratio of the thickness of the thick conductive layer tothe thin layer is 20:1.

The layer thicknesses are controlled by a standard quartz crystalthickness monitor in combination with a shutter.

In an alternative arrangement, the two-layer cathode described withreference to FIG. 1 is formed on the substrate, the at least one layerof a light-emissive organic material is formed over the cathode, and theanode formed over the at least one layer of light-emissive organicmaterial.

The thin layer of Li provides excellent electron injection and lowturn-on and operating voltage and, although diffusion of the Li fromlayer 4 into the PPV layer 3 with subsequent doping and quenching ofelectro-luminescence in the PPV is not prohibited, the limited thicknessand hence amount of material of layer 4 prevents excessive doping andelectro-luminescence quenching.

There thus has been described a device structure, and process offabrication thereof, for an OLED with an efficient low work functionelectron-injecting cathode with minimised risk of excessive doping ofthe organic layer(s) by the low work function cathode, and thereforeminimised risk of shorting of the device structure and quenching ofelectro-luminescence.

What is claimed is:
 1. An organic light-emitting device, comprising atleast one layer of a light-emissive organic material arranged between ananode and a cathode for the device, wherein the cathode comprises afirst layer of a conductive material which is an opaque metallic layerof high conductivity and a second layer of a conductive material havinga low work function arranged between the at least one layer of organicmaterial and the first layer of conductive material, wherein the secondlayer of conductive material is substantially thinner than the firstlayer of conductive material, having a thickness of at most 5 nm, andcomprises an elemental metal, an alloy or an intermetallic compoundhaving a work function of at most 3.7 eV.
 2. An organic light-emittingdevice according to claim 1, wherein the second layer of conductivematerial has a thickness in the range of from 0.5 to 2 nm, preferablyabout 0.5 nm.
 3. An organic light-emitting device according to claim 1,wherein the second layer of conductive material comprises an elementalmetal, an alloy or an intermetallic compound having a work function ofat most 3.2 eV.
 4. An organic light-emitting device according to any ofclaim 1, wherein the second layer of conductive material comprises oneof an alkali metal, an alkaline earth metal or a lanthanide, or an alloyor an intermetallic compound thereof.
 5. An organic light-emittingdevice according to claim 4, wherein the second layer of conductivematerial comprises one of Ca, K, Li, Sm or an Al—Li alloy.
 6. An organiclight-emitting device according to any of claim 1, wherein the ratio ofthicknesses of the first layer of conductive material to the secondlayer of conductive material is at least 20:1.
 7. An organiclight-emitting device according to claim 2, wherein the second layer ofconductive material comprises an elemental metal, an alloy or anintermetallic compound having a work function of at most 3.2 eV.
 8. Anorganic light-emitting device, comprising at least one layer of alight-emissive organic material arranged between an anode and a cathodefor the device, wherein the cathode comprises a first layer of aconductive material which is an opaque metallic layer of highconductivity and a second layer of a conductive material having a lowwork function arranged between the at least one layer of organicmaterial and the first layer of conductive material, wherein the secondlayer of conductive material is substantially thinner than the firstlayer of conductive material, having a thickness of at most 5 nm, andconsists essentially of an elemental metal having a work function of atmost 3.7 eV.
 9. An organic light-emitting device, comprising at leastone layer of a light-emissive organic material arranged between an anodeand a cathode for the device, wherein the cathode comprises a firstlayer of a conductive material which is an opaque metallic layer of highconductivity and a second layer of a conductive material having a lowwork function arranged between the at least one layer of organicmaterial and the first layer of conductive material, wherein the secondlayer of conductive material is substantially thinner than the firstlayer of conductive material, having a thickness of at most 5 nm, andcomprises an elemental metal, an alloy or an intermetallic compoundhaving a work function of at most 3.7 eV, wherein the ratio of thethickness of the first layer to the second layer is at least 20:1. 10.An organic light-emitting device, comprising at least one layer of alight-emissive organic material arranged between an anode and a cathodefor the device, wherein the cathode comprises a first layer of aconductive material which is an opaque metallic layer of highconductivity and a second layer of a conductive material having a lowwork function arranged between the at least one layer of organicmaterial and the first layer of conductive material, wherein the secondlayer of conductive material is substantially thinner than the firstlayer of conductive material, and the second layer has a thickness in arange of 0.5 to 2 nm.
 11. An organic light-emitting device, comprisingat least one layer of a light-emissive organic material arranged betweenan anode and a cathode for the device, wherein the cathode comprises afirst layer of a conductive material which is an opaque metallic layerof high conductivity and a second layer of a conductive material havinga low work function arranged between the at least one layer of organicmaterial and the first layer of conductive material, wherein the secondlayer of conductive material is substantially thinner than the firstlayer of conductive material, having a thickness in a range of 0.5 to 2nm, and comprises a material having a work function of at least 3.7 eV.